In this technical field, technologies disclosed in patent document 1 and patent document 2 are known.
In the invention disclosed in patent document 1, permanent magnet burying holes are provided at equal intervals for the number of magnetic poles in a rotor in the circumferential direction of a rotor core. In each permanent magnet burying hole, a permanent magnet is held, and each permanent magnet is disposed so that the magnetic pole surface different from an adjacent permanent magnet may form a magnetic pole part directed to the outer circumferential direction of the iron core. The outer circumferential curved surface of the rotor core opposite to each magnetic pole part is formed in an arc largest in the distance from a rotation center in the middle of the magnetic poles, and smallest in the distance from a rotation center between the magnetic poles.
FIG. 21 is a sectional view of a conventional permanent magnet buried type electric motor disclosed in patent document 2, and FIG. 22 is a partially magnified view of its rotor. The conventional configuration shown in these drawings is explained.
In the invention of patent document 2, as shown in FIG. 21 and FIG. 22, the outer circumferential portion of one pole of a permanent magnet of rotor 91 in which permanent magnet 95 is accommodated is composed of first outer circumferential portion P1 and second outer circumferential portion P2. First outer circumferential portion P1 intersects with a line (a d-axis) passing through center O of rotor 91 and the magnetic pole central part, and is formed in an arc shape of radius R centered on center O of rotor 91. Second outer circumferential portion P2 intersects with a line (a q-axis) passing through center O of rotor 91 and between the magnetic poles, and is formed in an arc shape of radius Rq centered on point Q positioned on the q-axis. The relation of R<Rq is established. Herein, first outer circumferential portion P1 and second outer circumferential portion P2 cross at intersection point γ. Thus, patent document 2 discloses the rotor having an outer circumferential shape composed of a plurality of arcs different in radius.
In the invention of patent document 1, however, the outer circumferential shape of the rotor corresponding to one magnetic pole is formed of a single arc. The magnetic flux is saturated in a portion (bridge portion) of a minimum distance between the magnet end part and the rotor outer circumference, and the magnetic flux supposed to flow into the bridge portion consequently flows into a portion of a larger distance between the magnet end part and the rotor outer circumference. It was hence difficult to adjust the direction of the magnetic flux in the arc portion, and also difficult to adjust the magnetic flux density distribution in the air gap part in a sinusoidal wave shape.
In the invention of patent document 2, the intersection point of second outer circumferential portion P2 and q-axis, center O of first outer circumferential portion P1, and center Q of second outer circumferential portion P2 are determined to be positioned on a same straight line. Hence, at intersection point γ between first outer circumferential portion P1 and second outer circumferential portion P2, a concave shape is formed. Therefore in the invention of patent document 2, the outer circumferential curved surface of the rotor core is not a curved surface changing smoothly near intersection point γ, and it was difficult to form the magnetic flux density distribution in the air gap part in a sinusoidal wave shape.
Patent document 1: Japanese Patent Unexamined Publication No. 2000-197292
Patent document 2: Japanese Patent Unexamined Publication No. 2004-260972